1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copy machine, a multifunction peripheral, and a facsimile machine, in which a toner image formed on an image bearing member is transferred onto an intermediate transfer member.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus, which is applied to a copy machine, a multifunction peripheral, a facsimile machine, etc., has a photosensitive drum (image bearing member) which carries a toner image thereon, and an intermediate transfer belt (intermediate transfer member). It is demanded by the market that the photosensitive drum and the intermediate transfer belt are driven such that surface speeds thereof are both constant.
This is because, first, in a case where time-synchronized exposure is employed as laser exposure for forming an electrostatic latent image on the photosensitive drum, variation in the surface speed of the photosensitive drum causes deviation of a laser irradiation position on the photosensitive drum from an original proper position thereon to be irradiated. Secondly, also in a process for transferring a toner image formed on the photosensitive drum onto the intermediate transfer belt (primary transfer), if there occurs an AC current-like variation in the difference of surface speed between the photosensitive drum and the intermediate transfer belt, the position of the toner image which is to be transferred onto the intermediate transfer belt deviates from the original proper position on which the toner image is to be transferred. This causes image defects on an image transferred onto a recording sheet, which are called color shift (positional displacement between respective colors) and banding (periodic positional displacement).
To overcome the above-mentioned problem, in driving the photosensitive drum and the intermediate transfer belt, a CPU performs feedback-control of the speed of a motor as a drive source, using a suitable one of various speed detection sensors and the like to thereby ensure highly-accurate speed constancy. As a drive motor, one employing a brushless DC motor (hereinafter referred to as the “BLDC motor”) is often used because of low-cost, quietness, and high efficiency.
Further, in recent years, as the speed feedback control using the BLDC motor, there is an example employing a method in which, for example, a rotary encoder is arranged on a drum shaft, and the CPU controls the BLDC motor to rotate the drum shaft at a constant speed.
However, in the above-mentioned speed feedback control, the CPU keeps track of the rotational speed of the drum shaft, but it does not keep track of the surface speed of the photosensitive drum. Therefore, it is difficult to control the surface speed of the photosensitive drum to a constant speed e.g. due to off-centering of the drum shaft and an error in accuracy of the diameter of the photosensitive drum. Such is also the case with the intermediate transfer belt, and the intermediate transfer belt suffers from the same problem e.g. due to off-centering of a shaft of a drive roller which drives the intermediate transfer belt, an error in accuracy of the diameter of the drive roller, and variation in thickness of the intermediate transfer belt.
Further, causes of the image defects include mutual interference caused by friction between the surface of the photosensitive drum and the transfer surface of the intermediate transfer belt. This is a problem that a speed variation occurring in one of the photosensitive drum and the intermediate transfer belt is transmitted to the other to have influence thereon.
In addition to these causes, as another cause, there may be mentioned an occurrence of a sporadic change in load on the intermediate transfer belt during transfer of a toner image carried on the intermediate transfer belt onto a recording sheet (secondary transfer), especially when the recording sheet is thick paper. This causes a high-frequency speed variation, and this speed variation may cause positional displacement in the primary transfer.
As described above, there are various causes of the image defects, and it is very difficult to eliminate all of the causes. To cope with this, as described in Japanese Patent Laid-Open Publication No. 2002-333752, there has been developed a technique in which an image transfer barrel (which corresponds to an intermediate transfer belt) causes an image barrel (which corresponds to a photosensitive drum) to be driven by friction therebetween (friction-driven).
This has the following merits: First, images on the photosensitive drums are transferred to form an image on the intermediate transfer belt, and hence by forming the image on the intermediate transfer belt with reference to respective positions on the photosensitive drums, the influence of irregular rotation of the photosensitive drums is offset. Further, secondly, even when the speed of the intermediate transfer belt is varied e.g. due to an impact generated upon entrance of a recording sheet into a secondary transfer section of the intermediate transfer belt, coincidence of respective images on the photosensitive drums and an image on the intermediate transfer belt can be ensured, which makes image defects difficult to be caused by the primary transfer.
However, as described in Japanese Patent Laid-Open Publication No. 2002-333752, to cause each photosensitive drum to be friction-driven in a proper fashion (without occurrence of a slip) by the intermediate transfer belt using a frictional force between the photosensitive drum and the intermediate transfer belt, it is required to increase transfer pressure applied by an associated primary transfer section. If transfer pressure applied by the primary transfer section is increased, load generated on the photosensitive drum and the intermediate transfer belt is increased, resulting in an increase in drive torque. This brings about a problem that a surface speed difference is likely to be generated between each photosensitive drum and the intermediate transfer belt, which causes image defects, such as color shift.